1. What's their entry average age?

The average age of entry into prostitution is 13 years old.

2. What are some common disease for those people?

Estimates of the prevalence of incest among prostitutes range from 65% to 90%. The sex of prostitution is physically harmful to women in prostitution. STDs (including HIV/AIDS, chlamydia, gonorrhea, herpes, human papilloma virus, and syphilis) are alarmingly high among women in prostitution. There's two result of prostitution, the physical effect of prostitution is unwanted pregnancy and miscarriage. The emotion one is the emotional health consequences of prostitution include severe trauma, stress, depression, anxiety, self-medication through alcohol and drug abuse; and eating disorders.

3. Is something force them to prostitution?

There's a lot of reasons force them to prostitution. 62% reported having been raped in prostitution. 73% reported having experienced physical assault in prostitution. 72% were currently or formerly homeless. 92% stated that they wanted to escape prostitution immediately. 

4. Did they have some training for this?

They do have some training for this, a professional prostitute, whether she works as a streetwalker, house prostitute, or call girl, can usually pick out one person in her past who "turned her out," that is, who taught her the basic techniques and rules of the prostitute's occupation.

5.Is this is legal?

In different country is different. In some country it is legal but some is not. It's depends on where are you from. A lot of governments turn a blind eye to prostitution, but that doesn't necessarily mean it's legal. But some country is legal, for example in Burkina Faso - The act of selling sex is legal, but soliciting prostitution is not. But in Japan - Japan says prostitution is illegal. But they have these "soaplands" all around the country where you can pay an independent contractor for a massage (so it’s not a brothel) and if you happen to hit if off really, really well and other things happen to go on, well, then that’s all fine and dandy.